This invention relates to a semiconductor laser chip and more particularly to a semiconductor laser chip having a buried heterostructure (BH structure).
Semiconductor laser chips having a variety of structures have been developed as one of the light sources for optical communication or for data processing units such as digital audio discs and the like. (Refer, for example, to "Nikkei Electronics", Sept. 14, 1981, pp. 138-151, published by Nikkei-McGrawHill.)
The Applicant of the present invention has developed a BH semiconductor laser chip as a semiconductor laser chip for optical communication and data processing, as described, for example, in "Hitachi Review", published by Hitachi Hyoron-Sha, Vol. 65, No. 10 (1983), pp. 39-48.
A semiconductor laser chip is constituted by a compound semiconductor of an InGaAsP system when used as a semiconductor laser chip for the optical communication, and by a compound semiconductor of a GaAlAs system when used for data processing. The laser chips of both the InGaAsP system and the GaAlAs system have closely analogous structures.
Here, a semiconductor laser chip for an InGaAsP system developed by the present Applicant prior to the present invention will be described briefly in order to assist in understanding the present invention. This earlier laser chip has a structure such as shown in FIG. 1. The laser chip is produced in the following manner. First of all, a substrate 1 made of n-type In (indium) - P (phosphorus) is prepared. The main plane (upper surface) of the substrate 1 is a (100) crystal plane. A buffer layer 2 consisting of n-type InP, an active layer 3 consisting of InGa (gallium)-As(arsenic), a cladding layer 4 consisting of p-type InP and a cap layer 5 consisting of p-type InGaAsP are sequentially formed on this (100) crystal plane by the liquid epitaxial method. Thereafter, this multi-layered grown layer is partly etched by a solution such as bromomethanol, forming strips 5 to 6 .mu.m wide. This striped portion is formed so as to extend in the &lt;110&gt; direction of the crystal (perpendicular to the (110) cleavage of the crystal), so that the portion ranging from the active layer 3, and the cladding layer 4 to the cap layer 5 forms an inverted triangle, i.e. the so-called "inverted mesa structure". The side surface of this inverted mesa structure, which will be hereinafter called the "inverted mesa plane" for the sake of convenience, is the (111) crystal plane, on which In appears. The width of the active layer 3 is determined solely by the depth from the surface of the crystal to the active layer 3 and by the width of an insulating layer mask which is disposed on the cap layer 5 during etching, but does not depend upon the etching condition. Therefore, it can be readily reproduced. The portion below the inverted mesa plane portion is a forward mesa structure, which describes gentle curves.
A blocking layer 6 consisting of p-type InP, a buried layer 7 consisting of n-type InP and a cap layer 8 consisting of n-type InGaAsP are formed at the portion which is recessed by etching. A Zn diffusion region 9 reaching the intermediate depth of the cladding layer 4 is formed on the surface of the mesa portion. An insulating film 10 is deposited on the main surface of the substrate 1 except for the electrode contact portions. An anode 11 consisting of an Au-type electrode is disposed on this insulating film 10 and the mesa portion. A cathode 12 consisting of the Au-type electrode 12 is disposed on the reverse side of the substrate 1.
An external force is applied by a diamond tool or the like to one end portion of the substrate 1 to form cleavage scratches with predetermined spaces between them along the cleavage plane of the crystal. Thereafter, bending stress is externally applied to the wafer to effect cleavage, and rectangular slices are formed. Each rectangular slice is scribed with a diamond tool or the like at predetermined intervals in a direction crossing the cleavage line at right angles, and the slices are cracked along the scribed lines, thereby providing a large number of laser chips.
A typical laser chip constructed in this fashion is 400 .mu.m wide, 300 .mu.m long and 100 .mu.m deep. When a predetermined voltage is applied across the anode 11 and the cathode 12, laser light is emitted from the edge surface (mirror surface) of the active layer which is 300 .mu.m long. This laser chip is used after being fixed to a support via the anode 11 or the cathode 12.
However, a laser chip of the kind described above often fails to emit the laser light and suffers from characteristic defects due to increased threshold current. As a result of studies, the inventor of this invention has found that one of the causes is the insufficient growth of the buried grown layer.
As shown in FIG. 2 (which is an enlarged sectional view showing schematically the part of the principal portion of the semiconductor laser chip shown in FIG. 1), a forward current (I.sub.F) flows, in principle, sequentially through the cladding layer 4, the active layer 3 and the buffer layer 2. In a defective laser chip in which the non-emission of the laser light and the increase in the threshold current such as described above occur, it has been found that a leakage current (I.sub.L)flows sequentially through the cladding layer 4, the buried layer 7, the ungrown region 13 in the blocking layer 6 (buried layer) and the buffer layer 2. Therefore, the forward current (I.sub.F) becomes the sum of the leakage current (I.sub.L) and a practical operating current (I.sub.d) that flows sequentially through the cladding layer 4, the active layer 3 and the buffer layer 2.
As a result, the threshold current value (I.sub.th) in the laser chip becomes excessively great or the operating current I.sub.d becomes excessively small because the leakage current becomes excessively great, so that laser oscillation does not occur. Such a laser chip is likely to cause screening problems and there are production problems as well. It has been found that the occurrence of an ungrown region in the blocking layer 6 develops because surface cleaning such as cleaning after etching is inadequate, and the buried layer (blocking layer 6) does not grow at this cleaning defect portion or the portion on which foreign matter has been deposited; consequently, an ungrown region 13 develops.
Another cause for defects will now be considered.
In the manufacture of laser chips, the wafer is cut in a grid pattern to obtain individual chips. At this time, fine cracks occur and foreign matter in the form of very fine impurities is likely to be deposited on the chip in such a manner as to bridge the p-n junction of the semiconductor laser chip. Since this foreign matter 17 is electrically conductive, it causes short-circuits, as shown in FIG. 1. Moreover, since the electrode material 11, 12 of this laser chip is gold (Au), therefore highly malleable, it cannot be cut without some stretching. Therefore, the electrode material is elongated when the wafer is cut off, and overhangs from the periphery of the laser chip, eventually causing a short-circuit.
The active layer portions forming the p-n junction and the boundary portion between the buffer layer 2 and the blocking layer 6 in this laser chip may be as shallow as 3 to 5 .mu.m, this being the distance from the main plane of the laser chip, and are exposed to the peripheral surface of the laser chip. As a result, the overhanging electrode material 18 is likely to bring the p-type region into contact with the n-type region, so that a short-circuit occurs and the withstand voltage drops.
If the deposition of foreign matter and the overhanging electrode material are found at inspection, the product is rejected. However, these problems sometimes do not occur until after inspection even though they are close to the p-n junction; consequently, defective chips may pass inspection. However, the foreign matter and the overhanging electrode material will become attached to the p-n junction portion eventually when the product is used in a working environment with a consequent malfunction of the chip.
The present invention aims to eliminate these problems of the prior art.